In a field of a head-up display apparatus of a vehicle, it is known that aberrations, which are generated in an optical system including a windshield of the vehicle, cause deformation of an image, which is displayed as a virtual image. One of the aberrations of the optical system is an aberration caused by a distortion of the image. A technique of reducing a two-dimensional deformation of the virtual image, which is caused by the distortion, is disclosed in, for example, JPH07-257225A, JPH10-149085A and JPH11-30764A.
Specifically, JPH07-257225A teaches a holographic display system, which includes a light emitting display means and a reflective hologram. The light emitting display means projects a light of a virtual image. The light, which is outputted from the light emitting display means, is projected onto the reflective hologram. A shape of the display image, which is projected by the light emitting display means, is pre-deformed (pre-distorted) in advance to compensate the deformation generated at the reflective hologram. Thus, it is possible to limit the two-dimensional deformation of the displayed virtual image.
JPH10-149085A teaches a holographic display apparatus, which includes a display, a light source and a hologram combiner. The display projects a light, which forms a display image. The light, which is outputted from the display, is projected onto the hologram combiner. A shape of the display image, which is projected by the display, is pre-deformed (pre-distorted) in advance to compensate the two-dimensional deformation of the image generated at the hologram combiner. Thus, it is possible to limit the two-dimensional deformation of the displayed image.
JPH11-30764A teaches a head-up display apparatus, which includes an image display surface and a half mirror. A light of an image is projected from the image display surface. The light, which is outputted from the image display surface, is projected onto the half mirror. The image, which is projected from the image display surface, is pre-deformed (pre-distorted) in advance to compensate a deformation of the image generated at the half mirror. Thus, it is possible to limit the two-dimensional deformation of the virtual image.
Lately, like in the case of JPH11-30764A, it is popular to use the head-up display apparatus, which projects a display image onto a concave windshield located on a front side of a viewer. In such a head-up display apparatus, in addition to the aberration caused by the two-dimensional deformation of the image, an aberration caused by a three-dimensional curvature of field occurs. Therefore, the displayed virtual image of the display image, which is viewed at a viewing area by a viewer, is deformed such that a distance between the viewing area of the viewer and the displayed virtual image decreases from a center portion of the displayed virtual image to an edge portion of the displayed virtual image.
In the head-up display apparatus of JPH11-30764A, the image display surface, which displays the display image, is formed as a planar surface. In addition, in general, it is difficult to change the shape of such an image display surface. Therefore, it is difficult to adjust a distance of a light path, which is from the image display surface to the windshield, and an imaging point of the virtual image. As a result, the three-dimensional deformation of the virtual image cannot be reduced. In the case where the three-dimensional deformation is generated in the virtual image, when the viewer moves his/her view point within the viewing area, a change in the shape and a change in the position occur in the virtual image of the display image. Therefore, the display quality of the display image, which is displayed as the virtual image, may possibly become insufficient.